Technical Field
The present disclosure relates to magnetic resonance imaging (MRI); more particularly, the present disclosure relates to method and apparatus for generating a flip angle distribution in MRI.
Description of Related Art
High-field MRI offers a great promise to generate images with high signal-to-noise ratio (SNR). Yet the major technical challenge is the inhomogeneous flip angle distribution when a volume RF coil is used for RF excitation. This artifact is due to the deleterious interaction between the dielectric properties of the sample and the radio-frequency field; consequently, when an object with the size approximating to the human head is imaged by high-field (>=3T) MRI, the flip angle distribution is spatially varying, where typically a larger flip angle at the center of the field-of-view (FOV) and a smaller flip angle at the periphery of the FOV. This causes images with a spatially dependent T1 contrast, which makes clinical diagnosis difficult.
Different methods for mitigating B1+ inhomogeneity have been proposed; for example, dedicated volume radio-frequency (RF) coils have been designed for use in high-field MRI; another method is using spatially selective RF excitation, wherein spatially selective RF excitation designs RF and gradient waveforms to form an inhomogeneous B1+ field in a volume coil, and finally generates a more homogeneous flip angle distribution. Alternatively, it has been suggested that flip angle distribution can become more homogeneous by using simultaneous RF excitation from multiple RF coils, such as RF shimming and transmit SENSE. Notably, parallel RF transmission (pTx) methods allow higher degree of freedom in RF pulse and gradient waveform design than RF shimming, because different RF pulse waveforms can be delivered to each RF coil independently; however, the challenges of parallel RF transmission method include the complexity of the RF electronics and coil construction in order to achieve simultaneous excitation, the necessity of accurate estimates of phases and amplitudes of the B1+ maps for each RF coil, and the specific absorption rate (SAR) management.
Recently, it has been demonstrated that nonlinear spatial encoding magnetic fields (SEMs) can be used in MRI spatial encoding in order to improve spatiotemporal resolution; preliminary studies using quadratic nonlinear SEMs for RF excitation and small FOV imaging have been reported. Nonlinear spatial encoding magnetic fields can also be used to mitigate the inhomogeneity of the flip angle distribution; under the small flip angle approximation, there are theories indicating how the spatial distribution of the flip angle is controlled by time-varying linear and nonlinear spatial encoding magnetic fields and RF pulse waveforms.
In summary, different techniques have been broached to improve the uniformity of the flip angle spatial distribution in high field MRI; however, at present time, there is no such technique which combines RF shimming and the usage of linear and nonlinear spatial encoding magnetic fields to achieve a homogeneous flip angle distribution.